System and method for increasing relative intensity between cathodes and  anodes of neurostimulation system

ABSTRACT

A method and neurostimulation system for providing therapy to a patient is provided. A plurality of electrodes is placed adjacent to tissue of the patient. The electrodes include first and second electrodes, with the first electrode having a first tissue contacting surface area and the second electrode having a second tissue contact surface area greater than the first tissue contacting surface area. Anodic electrical current is simultaneously sourced from one of the first and second electrodes to the tissue and while cathodic electrical current is sunk from the tissue to another of the first and second electrodes to provide the therapy to the patient.

RELATED APPLICATION DATA

The present application is a continuation of U.S. patent applicationSer. No. 13/937,035, filed Jul. 8, 2013, now issued as U.S. Pat. No.8,660,665, which is a continuation of U.S. patent application Ser. No.12/508,407, filed Jul. 23, 2009, now issued as U.S. Pat. No. 8,494,640,which claims the benefit under 35 U.S.C. §119 to U.S. provisional patentapplication Ser. No. 61/084,208, filed Jul. 28, 2008. The foregoingapplications are expressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to tissue stimulation systems, and moreparticularly, to a system and method for conditioning and stimulatingnerve fibers.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a widevariety of diseases and disorders. Pacemakers and Implantable CardiacDefibrillators (ICDs) have proven highly effective in the treatment of anumber of cardiac conditions (e.g., arrhythmias). Spinal CordStimulation (SCS) systems have long been accepted as a therapeuticmodality for the treatment of chronic pain syndromes, and theapplication of tissue stimulation has begun to expand to additionalapplications such as angina pectoralis and incontinence. Deep BrainStimulation (DBS) has also been applied therapeutically for well over adecade for the treatment of refractory chronic pain syndromes, and DBShas also recently been applied in additional areas such as movementdisorders and epilepsy. Further, in recent investigations, PeripheralNerve Stimulation (PNS) systems have demonstrated efficacy in thetreatment of chronic pain syndromes and incontinence, and a number ofadditional applications are currently under investigation. Furthermore,Functional Electrical Stimulation (FES) systems, such as the Freehandsystem by NeuroControl (Cleveland, Ohio), have been applied to restoresome functionality to paralyzed extremities in spinal cord injurypatients.

Each of these implantable neurostimulation systems typically includesone or more electrode carrying stimulation leads, which are implanted atthe desired stimulation site, and a neurostimulator implanted remotelyfrom the stimulation site, but coupled to the stimulation lead(s). Thus,electrical pulses can be delivered from the neurostimulator to thestimulation lead(s) to stimulate or activate a volume of neural tissue.In particular, electrical energy conveyed between at least one cathodicelectrode and at least one anodic electrodes creates an electricalfield, which when strong enough, depolarizes (or “stimulates”) theneurons beyond a threshold level, thereby inducing the firing of actionpotentials (APs) that propagate along the neural fibers.

Stimulation energy may be delivered to the electrodes during and afterthe lead placement process in order to verify that the electrodes arestimulating the target neural elements and to formulate the mosteffective stimulation regimen. The regimen will dictate which of theelectrodes are sourcing current pulses (anodes) and which of theelectrodes are sinking current pulses (cathodes) at any given time, aswell as the magnitude and duration of the current pulses. Thestimulation regimen will typically be one that provides stimulationenergy to all of the target tissue that must be stimulated in order toprovide the therapeutic benefit, yet minimizes the volume of non-targettissue that is stimulated. In the case of SCS, such a therapeuticbenefit is “paresthesia,” i.e., a tingling sensation that is effected bythe electrical stimuli applied through the electrodes.

While the electrical stimulation of neurons has generally beensuccessful in providing a therapeutic benefit to the patient, there areinstances where the target tissue is not directly adjacent to anelectrode and, because the electrical field strength decreasesexponentially with distance from the electrodes, a relatively strongelectrical field must be created to generate APs in the target neuralfibers. The electrical field may, however, also result in the generationof APs in the non-target neural fibers between the electrode and thetarget neural fibers. The generation of APs in the non-target neuralfibers may, in turn, lead to undesirable outcomes (e.g., discomfort orinvoluntary movements) for the patient. Because the target neural tissue(i.e., the tissue associated with the therapeutic effects) andnon-target neural tissue (i.e., the tissue associated with undesirableside effects) are often juxtaposed, therapeutically stimulating neuraltissue while preventing side effects may be difficult to achieve. In thecontext of SCS, there may be a few ways of eliminating, or at leastminimizing, the stimulation of non-target neural tissue.

For example, in the case where the electrode array is medio-laterallyaligned (i.e., the electrodes are arranged transversely to the neuralfibers of the spinal cord), it may be desirable to control the shape ofthe AP generating neural region of the spinal cord in order to preventthe generation of APs in non-target neural fibers. For example, toproduce the feeling of paresthesia without inducing involuntary motormovements within the patient, it is often desirable to preferentiallystimulate nerve fibers in the dorsal column (DC nerve fibers), whichprimarily include sensory nerve fibers, over nerve fibers in the dorsalroots (DR nerve fibers), which include both sensory nerve fibers andmotor reflex nerve fibers. While DC nerve fibers are the intendedtargets in conventional SCS, in fact, the DR nerve fibers often arerecruited first because of geometric, anatomical, and physiologicalreasons. For example, the DR nerve fibers have larger diameters than thelargest nearby DC nerve fibers, and thus, have a lower threshold atwhich they are excited. Other factors that contribute to the lowerthreshold needed to excite DR nerve fibers are the differentorientations of the DC nerve fibers and DR nerve fibers, the curvedshape of the DR nerve fibers, and the inhomogeneity and anisotropy ofthe surrounding medium at the entrance of the DR nerve fibers into thespinal cord. Thus, DR nerve fibers may still generate APs at lowervoltages than will nearby DC nerve fibers. As a result, the DC nervefibers that are desired to be stimulated have a lower probability to bestimulated than do the DR nerve fibers, and thus, the reflex motor nervefibers intermingled among the sensor nerve fibers of a dorsal root areoften recruited, leading to discomfort or muscle twitching, therebypreventing satisfactory paresthesia coverage.

For reasons such as these, it is often desirable to modify the thresholdat which neural tissue is activated in a manner that maximizesexcitation of the target neural tissue, while minimizing excitation ofthe non-target neural tissue; that is, to increase the DR/DC thresholdratio. This can be accomplished by sinking an electrical pulse to acathodic electrode located at the center of the spinal cord todepolarize the target tissue adjacent the cathodic electrode, therebycreating APs along the DC nerve fibers, while an electrical pulse can besourced to anodic electrodes on both sides of the cathodic electrode tohyperpolarize non-target tissue adjacent the anodic electrodes, therebyincreasing the threshold of the DR nerve fibers.

As another example, in the case where the electrode array isrostro-caudally aligned (i.e., the electrodes are arranged along theneural fibers of the spinal cord), it may be desirable to induce APs ina bundle of target nerve fibers, and to the extent that APs are inducedin bundle of non-target nerve fibers, block APs within the non-targetnerve fibers from reaching the brain or any other parts of the nervoussystem. In particular, an electrical pulse can be sunk to a cathodicelectrode to depolarize target tissue adjacent the cathodic electrode,thereby creating APs along a first bundle of nerve fibers, while anelectrical pulse can be sourced to one or more anodic electrodes aboveor below the cathodic electrode to hyperpolarize non-target tissueadjacent the anodic electrode(s), thereby blocking any APs along asecond bundle of nerve fibers that were inadvertently induced by thesink electrical pulse of the cathodic electrode.

Because the amount of electrical current that is sourced must equal theamount of electrical current that is sunk, the amount of sourcedelectrical current must be limited in order to minimize the adverseeffects that could potentially occur as a result of the increased amountof the sunk electrical current. For example, in the previously describedcase where the electrode array is rostro-caudally aligned, an increasein the electrical current sunk by the cathode as a result of an increasein the electrical current sourced by the anodes(s) may result in thegeneration of APs in non-target nerve fibers that are not blocked by thesourced electrical current. In the previously described case where theelectrode array is medio-laterally aligned, an increase in theelectrical current sunk by the cathode as a result of an increase in theelectrical current sourced by the anodes may result in the generation ofAPs in non-target DC nerve fibers.

To limit the amount of current sunk by a cathode, it is known toredistribute some of the cathodic current to a large surface area, suchas the case of the IPG. Such a technique is described in U.S. patentapplication Ser. No. 11/300,963, entitled “Apparatus and Methods forStimulating Tissue,” which is expressly incorporated herein byreference. By distributing the cathodic current to a surface area thathas no, or very little, effect on the neural tissue, the magnitude ofthe electrical pulses sourced by the anodes can be increased whileavoiding a commensurate increase in the magnitude of the electricalpulses sunk to the cathode that is adjacent the neural tissue. In thismanner, any adverse effects that may otherwise occur as a result of anincrease in the electrical current sunk to the cathodic electrode, andthus conveyed through the neural tissue adjacent the cathodic electrode,can be minimized.

While this electrical current redistribution technique is beneficial, itcan only be implemented within an IPG that has independent current orvoltage sources for the electrodes. That is, an IPG with a singlecurrent or voltage source provides no means for redistributing aselected amount of cathode current to the IPG case.

There, thus, remains a need for an alternative neurostimulation methodand system that minimizes any adverse effects that may result in anincrease in cathodic current when the anodic current is increased.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, a method ofproviding therapy to a patient is provided. The method comprises placinga plurality of electrodes adjacent to tissue of the patient. Theelectrodes include first and second electrodes, with the first electrodehaving a first tissue contacting surface area and the second electrodehaving a second tissue contact surface area greater than (e.g., at leasttwice) the first tissue contacting surface area.

The method further comprises simultaneously sourcing anodic electricalcurrent from one of the first and second electrodes (e.g., the firstelectrode) to the tissue and sinking cathodic electrical current fromthe tissue to another of the first and second electrodes (e.g., thesecond electrode) to provide the therapy to the patient. In one method,the anodic electrical current and cathodic electrical current take theform of electrical pulses. In another method, the size disparity betweenthe first and second tissue contacting surfaces causes the currentdensity on the first tissue contacting surface to be greater than thecurrent density on the second tissue contacting surface.

The tissue to which the electrodes are placed adjacent can be, e.g.,spinal cord tissue. In one method, the electrodes are arrangedmedio-laterally along the spinal cord tissue. In this case, the secondelectrode can be adjacent to dorsal column neural fibers of the spinalcord tissue, the first electrode can be adjacent to dorsal root neuralfibers of the spinal cord tissue, the sunk cathodic electrical currentcan generate action potentials in the dorsal column neural fibers of thespinal cord tissue, and the sourced anodic electrical current canincrease the action potential threshold of the dorsal root neuralfibers. In another exemplary method, the electrodes are arrangedrostro-caudally along the spinal cord tissue. In this case, the secondelectrode can be a first distance from the first neural fiber bundle anda second greater distance from the second neural fiber bundle, the sunkcathodic electrical current can generate action potentials in the firstand second neural fibers bundles, and the sourced anodic electricalcurrent can block at least some of the action potentials in the firstneural fiber bundle.

In accordance with a second aspect of the present inventions, aneurostimulation system is provided. The neurostimulation systemcomprises a plurality of electrodes configured for being placed adjacentto tissue of a patient. The electrodes include first and secondelectrodes, with the first electrode having a first tissue contactingsurface area and the second electrode having a second tissue contactsurface area greater than (e.g., at least twice) the first tissuecontacting surface area.

The neurostimulation system further comprises output stimulationcircuitry coupled to the plurality of electrodes. The output stimulationcircuitry is configured for sourcing anodic electrical current to one ofthe first and second electrodes (e.g., the first electrode) and sinkingcathodic electrical current from another of the first and secondelectrodes (e.g., the second electrode) to provide therapy to thepatient. In one embodiment, the anodic electrical current and cathodicelectrical current take the form of electrical pulses. In anotherembodiment, the size disparity between the first and second tissuecontacting surfaces is such that the output stimulation circuitry isconfigured for generating a current density on the first tissuecontacting surface that is greater than the current density on thesecond tissue contacting surface.

In one embodiment, the neurostimulation system further comprises a lead(e.g., spinal cord stimulation lead) that carries the electrodes. Thelead may be, e.g., an in-line lead, in which case, the electrodes arearranged in a single column along the axis of the in-line lead, or apaddle lead, in which case, three of the electrodes may be arrangedalong a line transverse to the axis of the paddle lead.

In accordance with a third aspect of the present inventions, aneurostimulation lead is provided. The neurostimulation lead comprisesan elongated lead body and a plurality of electrodes carried by the leadbody. The lead body may be configured for, e.g., being placed adjacentspinal cord tissue. In one embodiment, the electrodes are ringelectrodes disposed in a single column around the lead body. In anotherembodiment, the neurostimulation lead further comprises a paddledisposed on the lead body, in which case, three of the electrodes may bedisposed on the paddle along a line transverse to the to the axis of thelead body. The electrodes include first and second electrodes, with thefirst electrode having a first tissue contacting surface area and thesecond electrode having a second tissue contact surface area greaterthan (e.g., at least twice) the first tissue contacting surface area. Inone embodiment, the electrodes comprises three columns of electrodes, acenter one of the three columns of electrodes has a first total tissuecontacting surface, and remaining ones of the three columns ofelectrodes has a second total tissue contact surface that is less thanthe first total tissue contacting surface.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is plan view of one embodiment of a spinal cord stimulation (SCS)system arranged in accordance with the present inventions;

FIG. 2 is a plan view of an implantable pulse generator (IPG) and oneembodiment of a stimulation lead used in the SCS system of FIG. 1;

FIG. 3 is a plan view of an implantable pulse generator (IPG) andanother embodiment of a stimulation lead used in the SCS system of FIG.1;

FIG. 4 is a block diagram of the internal components of the IPG of FIG.1;

FIG. 5 is a plan view of the SCS system of FIG. 1 in use with a patient;

FIG. 6 is a perspective view of the electrodes of the stimulation leadof FIG. 2 medio-laterally located over the spinal cord of a patient;

FIG. 7 is a perspective view of the electrodes of the stimulation leadof FIG. 3 rostro-caudally located over the spinal cord of a patient;

FIG. 8 is a cross-section diagram of a spinal cord, particularlyillustrating a locus of stimulation induced by a prior art medio-lateralelectrode arrangement;

FIG. 9 is a cross-section diagram of a spinal cord, particularlyillustrating a locus of stimulation induced by the medio-lateralelectrode arrangement of FIG. 7;

FIG. 10 is a graph of the changes in neural fiber transmembranepotential in first and second fibers bundles induced by a prior artrostro-caudal electrode arrangement; and

FIG. 11 is a graph of the changes in neural fiber transmembranepotential in first and second fibers bundles induced by therostro-caudal electrode arrangement of FIG. 8.

DETAILED DESCRIPTION OF THE EMBODIMENTS

At the outset, it is noted that the present invention may be used withan implantable pulse generator (IPG), radio frequency (RF) transmitter,or similar electrical stimulator, that may be used as a component ofnumerous different types of stimulation systems. The description thatfollows relates to a spinal cord stimulation (SCS) system. However, itis to be understood that the while the invention lends itself well toapplications in SCS, the invention, in its broadest aspects, may not beso limited. Rather, the invention may be used with any type ofimplantable electrical circuitry used to stimulate tissue. For example,the present invention may be used as part of a pacemaker, adefibrillator, a cochlear stimulator, a retinal stimulator, a stimulatorconfigured to produce coordinated limb movement, a cortical stimulator,a deep brain stimulator, a peripheral nerve stimulator, or in any otherneural stimulator configured to treat urinary incontinence, sleep apnea,shoulder sublaxation, etc.

Turning first to FIG. 1, an exemplary SCS system 10 generally comprisesat least one implantable stimulation lead 12, an implantable pulsegenerator (IPG) 14 (or alternatively RF receiver-stimulator), anexternal remote control RC 16, a Clinician's Programmer (CP) 18, anExternal Trial Stimulator (ETS) 20, and an external charger 22.

The IPG 14 is physically connected via one or more lead extensions 24 tothe stimulation lead 12, which carries a plurality of electrodes 26arranged in an array. The stimulation lead 12 is illustrated as asurgical paddle lead in FIG. 1, although as will be described in furtherdetail below, one or more percutaneous stimulation leads can be used inplace of the surgical paddle lead 12. As will also be described infurther detail below, the IPG 14 includes pulse generation circuitrythat delivers electrical stimulation energy in the form of a pulsedelectrical waveform (i.e., a temporal series of electrical pulses) tothe electrode array 26 in accordance with a set of stimulationparameters.

The ETS 20, which has similar pulse generation circuitry as the IPG 14,also provides electrical stimulation energy to the electrode array 26 inaccordance with a set of stimulation parameters. The major differencebetween the ETS 20 and the IPG 14 is that the ETS 20 is anon-implantable device that is used on a trial basis after thestimulation lead 12 has been implanted and prior to implantation of theIPG 14, to test the effectiveness of the stimulation that is to beprovided.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andstimulation lead 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation programs after implantation. Oncethe IPG 14 has been programmed, and its power source has been charged orotherwise replenished, the IPG 14 may function as programmed without theRC 16 being present.

The CP 18 provides clinician detailed stimulation parameters forprogramming the IPG 14 and ETS 20 in the operating room and in follow-upsessions. The CP 18 may perform this function by indirectlycommunicating with the IPG 14 or ETS 20, through the RC 16, via an IRcommunications link 36. Alternatively, the CP 18 may directlycommunicate with the IPG 14 or ETS 20 via an RF communications link (notshown). The external charger 22 is a portable device used totranscutaneously charge the IPG 14 via an inductive link 38.

For purposes of brevity, the details of the RC 16, CP 18, ETS 20, andexternal charger 22 will not be described herein. Details of exemplaryembodiments of these devices are disclosed in U.S. Pat. No. 6,895,280,which is expressly incorporated herein by reference.

Referring further to FIG. 2, the IPG 14 comprises an outer case 15 forhousing the electronic and other components (described in further detailbelow), and a connector 17 in which the proximal end of the stimulationlead 12 mates in a manner that electrically couples the electrodes 26 tothe electronics within the outer case 15. The outer case 15 is composedof an electrically conductive, biocompatible material, such as titanium,and forms a hermetically sealed compartment wherein the internalelectronics are protected from the body tissue and fluids. In somecases, the outer case 15 serves as an electrode.

In the embodiment illustrated in FIG. 2, the stimulation lead 12 is asurgical paddle lead that comprises an elongated body 40 having aproximal end 42 and a distal end 44, and a paddle-shaped membrane 46formed at the distal end 44 of the lead body 40. In an alternativeembodiment, the stimulation lead 12 may include multiple elongatedbodies, in which case, the paddle-shaped membrane 46 may be formed atthe distal ends of the elongated bodies. The lead body 40 may, e.g.,have a diameter within the range of 0.03 inches to 0.07 inches and alength within the range of 30 cm to 90 cm for spinal cord stimulationapplications. Each lead body 40 may be composed of a suitableelectrically insulative material, such as, a polymer (e.g., polyurethaneor silicone), and may be extruded from as a unibody construction. Thepaddle-shaped membrane 46 is composed of an electrically insulativematerial, such as silicone.

The stimulation lead 12 further comprises a plurality of terminals (notshown) mounted to the proximal end 42 of the lead body 40 and theplurality of electrodes 16 mounted on one side of the paddle-shapedmembrane 46 in a two-dimensional arrangement. In the illustratedembodiment, the electrodes 26 are arranged in three columns on one sideof the paddle-shaped membrane 46 along the axis of the stimulation lead12, with the electrodes in the center column being labeled E1-E5, theelectrodes in one of the lateral columns (right column when the lead 12is introduced into the patient in the rostral direction) being labeledE6-E11, and the electrodes in the other of the lateral columns (leftcolumn when the lead 12 is introduced into the patient in the rostraldirection) being labeled E12-E17. Although the stimulation lead 12 isshown as having seventeen electrodes 26, the number of electrodes may beany number suitable for the application in which the stimulation lead 12is intended to be used (e.g., three, five, eight, eleven, etc.). Each ofthe electrodes 26 takes the form of a disk composed of an electricallyconductive, non-corrosive, material, such as, e.g., platinum, titanium,stainless steel, or alloys thereof.

The stimulation lead 12 also includes a plurality of electricalconductors (not shown) extending through the lead body 40 and connectedbetween the respective terminals (not shown) and electrodes 26 usingsuitable means, such as welding, thereby electrically coupling theproximally-located terminals with the distally-located electrodes 26. Inthe case where the stimulation lead 12 includes multiple elongatedbodies, the proximally-located terminals on each lead body will beelectrically coupled to a specific column of electrodes 26 located onthe paddle-shaped membrane 46 (in this case, the conductors within afirst lead body would be coupled to electrodes E1-E5, the conductorswithin a second lead body would be coupled to electrodes E6-E11, and theconductors within a third lead body would be coupled to electrodesE12-E17).

Further details regarding the construction and method of manufacture ofpaddle leads are disclosed in U.S. patent application Ser. No.11/319,291, entitled “Stimulator Leads and Methods for LeadFabrication,” the disclosure of which is expressly incorporated hereinby reference.

Significantly, the electrodes 26 include a first set of smallerelectrodes 26(a), each of which has a first tissue contacting surfacearea, and a second set of larger electrodes 26(b), each of which has asecond tissue contacting surface area that is greater than the firsttissue contacting surface area. In this manner, as will be described infurther detail below, the electrical current density at the largerelectrodes 26(b) will be decreased relative to the electrical currentdensity at the smaller electrodes 26(a), or conversely, the electricalcurrent density at the smaller electrodes 26(a) will be increasedrelative to the electrical current density at the larger electrodes26(b).

Preferably, the second tissue contacting surface area is at least twiceas large as the first tissue contacting surface area. In the illustratedembodiment, the second tissue contacting surface area is greater thanfive times as large as the first tissue contacting surface. In theillustrated embodiment, the center column includes the larger electrodes26(b), while the two lateral columns include the smaller electrodes26(a). Each larger electrode 26(b) is centered between four smallerelectrodes 26(a). Although the number of smaller electrodes 26(a) isgreater than the number of larger electrodes 26(b), in the illustratedembodiment, the total surface area of the larger electrodes 26(b) isgreater than the total surface area of the smaller electrodes 26(a).

In an alternative embodiment illustrated in FIG. 3, a percutaneousstimulation lead 48 can be used in the SCS system 10 instead of thesurgical paddle lead 12. Although only one percutaneous stimulation lead48 is shown, multiple percutaneous stimulation leads (e.g., two), can beused with the SCS system 10. The stimulation lead 48 includes anelongated lead body 50 having a proximal end 52 and a distal end 54. Thelead body 50 may, e.g., have a diameter within the range of 0.03 inchesto 0.07 inches and a length within the range of 30 cm to 90 cm forspinal cord stimulation applications. The lead body 50 may be composedof a suitable electrically insulative material, such as, a polymer(e.g., polyurethane or silicone), and may be extruded from as a unibodyconstruction.

The stimulation lead 48 further comprises a plurality of terminals (notshown) mounted to the proximal end 52 of the lead body 50 and aplurality of in-line electrodes 56 (in this case, eight electrodesE1-E8) mounted to the distal end 54 of the lead body 50. Although thestimulation lead 48 is shown as having eight electrodes 56 (and thus,eight corresponding terminals), the number of electrodes may be anynumber suitable for the application in which the stimulation lead 48 isintended to be used (e.g., two, four, sixteen, etc.). Each of theelectrodes 56 takes the form of a cylindrical ring element composed ofan electrically conductive, non-corrosive, material, such as, e.g.,platinum, titanium, stainless steel, or alloys thereof, which iscircumferentially disposed about the lead body 50.

The stimulation lead 48 also includes a plurality of electricalconductors (not shown) extending within the lead body 50 and connectedbetween the respective terminals (not shown) and electrodes 56 usingsuitable means, such as welding, thereby electrically coupling theproximally-located terminals with the distally-located electrodes 56.The stimulation lead 48 further includes a central lumen (not shown)that may be used to accept an insertion stylet (described in furtherdetail below) to facilitate lead implantation.

Further details describing the construction and method of manufacturingpercutaneous stimulation leads are disclosed in U.S. patent applicationSer. No. 11/689,918, entitled “Lead Assembly and Method of Making Same,”and U.S. patent application Ser. No. 11/565,547, entitled “CylindricalMulti-Contact Electrode Lead for Neural Stimulation and Method of MakingSame,” the disclosures of which are expressly incorporated herein byreference.

Significantly, the electrodes 56 include a first set of smallerelectrodes 56(a), each of which has a first tissue contacting surfacearea, and a second set of larger electrodes 56(b), each of which has asecond tissue contacting surface area that is greater than the firsttissue contacting surface area. In the same manner described above withrespect to the electrodes 26, the electrical current density at thelarger electrodes 56(b) will be decreased relative to the electricalcurrent density at the smaller electrodes 56(a), or conversely, theelectrical current density at the smaller electrodes 56(a) will beincreased relative to the electrical current density at the largerelectrodes 56(b). Again, the second tissue contacting surface area is atleast twice as large as the first tissue contacting surface area. In theillustrated embodiment, equal numbers of smaller electrodes 56(a) andlarger electrodes 56(b) extend along the axis of the stimulation lead 48in an alternating fashion.

As will be described in further detail below, the IPG 14 includes pulsegeneration circuitry that provides electrical conditioning andstimulation energy to the electrodes 26 (or alternatively the electrodes56) in accordance with a set of parameters. Such parameters may compriseelectrode combinations, which define the electrodes that are activatedas anodes (positive), cathodes (negative), and turned off (zero), andelectrical pulse parameters, which define the pulse amplitude (measuredin milliamps or volts depending on whether the IPG 14 supplies constantcurrent or constant voltage to the electrodes), pulse duration (measuredin microseconds), and pulse rate (measured in pulses per second).

With respect to the pulse patterns provided during operation of the SCSsystem 10, electrodes that are selected to transmit or receiveelectrical energy are referred to herein as “activated,” whileelectrodes that are not selected to transmit or receive electricalenergy are referred to herein as “non-activated.” Electrical energydelivery will occur between two (or more) electrodes, one of which maybe the IPG case, so that the electrical current has a path from theenergy source contained within the IPG case to the tissue and a sinkpath from the tissue to the energy source contained within the case.Electrical energy may be transmitted to the tissue in a monopolar ormultipolar (e.g., bipolar, tripolar, etc.) fashion.

Monopolar delivery occurs when a selected one or more of the leadelectrodes is activated along with the case of the IPG 14, so thatelectrical energy is transmitted between the selected electrode andcase. Monopolar delivery may also occur when one or more of the leadelectrodes are activated along with a large group of lead electrodeslocated remotely from the one more lead electrodes so as to create amonopolar effect; that is, electrical energy is conveyed from the one ormore lead electrodes in a relatively isotropic manner. Bipolar deliveryoccurs when two of the lead electrodes are activated as anode andcathode, so that electrical energy is transmitted between the selectedelectrodes. Tripolar delivery occurs when three of the lead electrodesare activated, two as anodes and the remaining one as a cathode, or twoas cathodes and the remaining one as an anode.

Turning next to FIG. 4, the main internal components of the IPG 14 willnow be described. The IPG 14 includes analog output circuitry 60 capableof individually generating electrical stimulation pulses via capacitorsC1-Cn at the electrodes 26 (or alternatively the electrodes 56) (E1-En)of specified amplitude under control of control logic 62 over data bus64. The duration of the electrical stimulation (i.e., the width of thestimulation pulses), is controlled by the timer logic 66.

Because the present invention lends itself well to simplistic electricalenergy delivery systems, the analog output circuitry 60 comprises one ormore current or voltage sources 68. The one or more current or voltagesources 68 can be, e.g., either a single current source for sourcing andsinking electrical pulses of a specified and known amperage to and fromthe electrodes 26, or a single voltage source for sourcing and sinkingelectrical pulses of a specified and known voltage to or from theelectrodes 26. However, in alternative embodiments, the analog outputcircuitry 60 may comprise independently controlled current sources forsourcing and sinking electrical pulses of a specified and known amperageto or from the electrodes 26, or independently controlled voltagesources for sourcing or sinking electrical pulses of a specified andknown voltage to or from the electrodes 26.

In any event, the analog output circuitry 60 includes a switch matrix 69coupled between the electrodes 26 and the power source, such thatselected ones of the electrodes 26 can be configured as cathodes (bycoupling them to a negative terminal of the source(s) 68) and selectedones of the electrodes 26 can be configured as anodes (by coupling themto a positive terminal of the source(s) 68).

The IPG 14 further comprises monitoring circuitry 70 for monitoring thestatus of various nodes or other points 72 throughout the IPG 14, e.g.,power supply voltages, temperature, battery voltage, and the like. TheIPG 14 further comprises processing circuitry in the form of amicrocontroller 74 that controls the control logic 62 over data bus 76,and obtains status data from the monitoring circuitry 70 via data bus78. The IPG 14 additionally controls the timer logic 66. The IPG 14further comprises memory 80 and oscillator and clock circuit 82 coupledto the microcontroller 74. The microcontroller 74, in combination withthe memory 80 and oscillator and clock circuit 82, thus comprise amicroprocessor system that carries out a program function in accordancewith a suitable program stored in the memory 80. Alternatively, for someapplications, the function provided by the microprocessor system may becarried out by a suitable state machine.

Thus, the microcontroller 74 generates the necessary control and statussignals, which allow the microcontroller 74 to control the operation ofthe IPG 14 in accordance with a selected operating program andparameters. In controlling the operation of the IPG 14, themicrocontroller 74 is able to individually generate electrical pulses atthe electrodes 26 using the analog output circuitry 60, in combinationwith the control logic 62 and timer logic 66, thereby allowing eachelectrode 26 to be paired or grouped with other electrodes 26, includingthe monopolar case electrode, and to control the polarity, amplitude,rate, and pulse width through which the current stimulus pulses areprovided.

The IPG 14 further comprises an alternating current (AC) receiving coil84 for receiving programming data (e.g., the operating program and/orstimulation parameters) from the RC 16 in an appropriate modulatedcarrier signal, and charging and forward telemetry circuitry 86 fordemodulating the carrier signal it receives through the AC receivingcoil 84 to recover the programming data, which programming data is thenstored within the memory 80, or within other memory elements (not shown)distributed throughout the IPG 14.

The IPG 14 further comprises back telemetry circuitry 88 and analternating current (AC) transmission coil 90 for sending informationaldata sensed through the monitoring circuitry 70 to the RC 16. The backtelemetry features of the IPG 14 also allow its status to be checked.For example, any changes made to the stimulation parameters areconfirmed through back telemetry, thereby assuring that such changeshave been correctly received and implemented within the IPG 14.Moreover, upon interrogation by the RC 16, all programmable settingsstored within the IPG 14 may be uploaded to the RC 16.

The IPG 14 further comprises a rechargeable power source 92 and powercircuits 94 for providing the operating power to the IPG 14. Therechargeable power source 92 may, e.g., comprise a lithium-ion orlithium-ion polymer battery. The rechargeable battery 92 provides anunregulated voltage to the power circuits 94. The power circuits 94, inturn, generate the various voltages 96, some of which are regulated andsome of which are not, as needed by the various circuits located withinthe IPG 14. The rechargeable power source 92 is recharged usingrectified AC power (or DC power converted from AC power through othermeans, e.g., efficient AC-to-DC converter circuits, also known as“inverter circuits”) received by the AC receiving coil 84. To rechargethe power source 92, an external charger (not shown), which generatesthe AC magnetic field, is placed against, or otherwise adjacent, to thepatient's skin over the implanted IPG 14. The AC magnetic field emittedby the external charger induces AC currents in the AC receiving coil 84.The charging and forward telemetry circuitry 86 rectifies the AC currentto produce DC current, which is used to charge the power source 92.While the AC receiving coil 84 is described as being used for bothwirelessly receiving communications (e.g., programming and control data)and charging energy from the external device, it should be appreciatedthat the AC receiving coil 84 can be arranged as a dedicated chargingcoil, while another coil, such as coil 90, can be used forbi-directional telemetry.

It should be noted that the diagram of FIG. 4 is functional only, and isnot intended to be limiting. Those of skill in the art, given thedescriptions presented herein, should be able to readily fashionnumerous types of IPG circuits, or equivalent circuits, that carry outthe functions indicated and described. It should be noted that ratherthan an IPG, the SCS system 10 may alternatively utilize an implantablereceiver-stimulator (not shown) connected to the stimulation lead 12. Inthis case, the power source, e.g., a battery, for powering the implantedreceiver, as well as control circuitry to command thereceiver-stimulator, will be contained in an external controllerinductively coupled to the receiver-stimulator via an electromagneticlink. Data/power signals are transcutaneously coupled from acable-connected transmission coil placed over the implantedreceiver-stimulator. The implanted receiver-stimulator receives thesignal and generates the stimulation in accordance with the controlsignals.

Referring to FIG. 5, the stimulation lead 12 (or alternatively thestimulation lead 48) is implanted within the spinal column 100 of apatient 98. The preferred placement of the stimulation lead 12 isadjacent, i.e., resting upon, the spinal cord area to be stimulated. Dueto the lack of space near the location where the stimulation lead 12exit the spinal column 100, the IPG 14 is generally implanted in asurgically-made pocket either in the abdomen or above the buttocks. TheIPG 14 may, of course, also be implanted in other locations of thepatient's body. The lead extension 24 facilitates locating the IPG 14away from the exit point of the stimulation lead 12. After implantation,the IPG 14 is used to provide the therapeutic stimulation under controlof the patient. The electrodes 26 may be arranged medio-laterally withrespect to the spinal cord, or alternatively, the electrodes 56 may bearranged rostro-caudally with respect to the spinal cord.

For example, as shown in FIG. 6, the surgical lead 12 illustrated inFIG. 2 can be used to arrange five electrodes 26 (one center electrodeE_(C) located over the center of the dorsal column DC nerve fibers, twoleft electrodes E_(L) laterally placed from the center of the DC nervefibers adjacent the left dorsal root DR nerve fibers, and two rightelectrodes E_(R) laterally placed from the center of the dorsal columnDC nerve fibers adjacent the right dorsal root DR nerve fibers)transverse to the axis of the spinal cord SC (medio-laterally). As thereshown, the larger electrode 26(b) is the center electrode E_(C), whilethe smaller electrodes 26(a) are the left and right electrodes E_(L),E_(R).

As another example, as shown in FIG. 7, the percutaneous lead 48illustrated in FIG. 3 can be used to arrange three electrodes 26 (anupper (or rostral) electrode E_(U), a center electrode E_(C), and alower (or caudal) electrode E_(L)) along the axis of the spinal cord SC(rostro-caudally) over the dorsal column DC nerve fibers. As thereshown, the larger electrode 56(b) is the center electrode E_(C), whilethe smaller electrodes 56(a) are the upper and lower electrodes E_(U),E_(L).

The SCS system 10 has application in a wide variety of electricalstimulation regimens.

For example, neurostimulation regimens that use the surgical paddle lead12 to medio-laterally arrange the electrodes 26 in the mannerillustrated in FIG. 6 can be used to shape of the AP generating neuralregion of the spinal cord in order to prevent the generation of APs innon-target neural fibers. As shown in FIGS. 8 and 9, the centerelectrode E_(C) is placed over the dorsal column DC nerve fibers, whilethe two left electrodes E_(L) (only one shown) and the two rightelectrodes E_(R) (only one shown) are respectively placed over thedorsal root DR nerve fibers on both sides of the dorsal column DC nervefibers.

A conventional stimulation regimen that uses uniformly sized electrodeswill serve as a reference for the stimulation regimens performed inaccordance with the present inventions, and will thus be initiallydescribed with reference to FIG. 8. In this conventional stimulationregimen, the left and right electrodes E_(L) (2 each) and E_(R) (2 each)are activated as anodes and the center electrode E_(C) is activated as acathode. In the illustrated embodiment, the four electrodes E_(L), E_(R)are each sourcing 25% of the total current (e.g., 1 mA each) the centerelectrode E_(C) is sinking 100% of the total current (e.g., 4 mA). Thecombination of the hyperpolarizing electric fields generated by the leftand right electrodes E_(L), E_(R) and the depolarizing electric fieldgenerated by the center electrode E_(C) results in an area within thedorsal column DC that is at or above the depolarization threshold. Thisarea, which has an overall depth and width, is the locus of stimulationLOS.

In the conventional stimulation regimen described above, it is desirablethat the locus of stimulation LOS be as narrow as possible withoutincreasing the depth of the LOS, thereby stimulating target nerve fiberswithin the dorsal column DC, while preventing stimulation of non-targetnerve fibers within the dorsal roots DR. This would require an increasein the hyperpolarizing electrical field generated by the left and rightelectrodes E_(L), E_(R) over that illustrated in FIG. 8. That is,strengthening of the hyperpolarizing electric fields created by theelectrodes E_(L), E_(R) tends to result in a narrowing the locus ofstimulation LOS because it weakens the lateral edges of the depolarizingelectric field created by the center electrode E_(C). However, thisnecessarily may result in an increase in the current sunk by the centerelectrode E_(C), thereby increasing the depth of the locus ofstimulation LOS, which may lead to undesirable outcomes (e.g.,discomfort or undesirable reflexive activity).

The SCS system 10 may be used to solve this problem by effectivelyincreasing the AP threshold of the dorsal root DR nerve fibers relativeto the AP threshold of the dorsal column DC nerve fibers. As illustratedin FIG. 9, one example of a stimulation regimen in accordance with apresent invention involves creating a locus of stimulation LOS that hasa smaller width and the same depth. Here, in the same manner describedabove with respect to FIG. 8, the left and right electrodes E_(L), E_(R)are activated as anodes and the center electrode E_(C) is activated as acathode. However, the amount of current sourced at the left and rightelectrodes E_(L), E_(R) should be sufficient to create a hyperpolarizingelectric field that is strong enough to narrow the locus of stimulationLOS to the smaller width. For example, the current sourced at the leftand right electrodes E_(L), E_(R) may be increased (e.g., 4-8 mA each)in order to strengthen the hyperpolarizing electric fields.

Notably, if the sizes of the electrodes E_(L), E_(R), E_(C) were thesame, sinking all of the current sourced by the left and rightelectrodes E_(L), E_(R) at the center electrode E_(C) could result in adepolarizing electric field that would undesirably increase the depth ofthe locus of stimulation LOS. However, because center electrode E_(C)has an increased tissue contacting surface area, the decreased currentdensity will compensate for the increased current at the centerelectrode E_(C), thereby allowing the intensity of the depolarizingelectric field created by the center electrode E_(C) to be reduced to alevel that does not increase the depth of the locus of stimulation LOScompared to that illustrated in FIG. 8.

Alternatively, rather the narrowing the locus of stimulation LOS in bothdirections, the locus of stimulation LOS may be narrowed in only onedirection. Here, only one of left and right electrodes E_(L), E_(R) isactivated as an anode. In this case, 100% of the total current is beingsourced at the left electrode E_(L) or right electrode E_(R), and 100%of the total current is being sunk at the center electrode E_(C).

As another example, neurostimulation regimens that use the percutaneouslead 48 to rostro-caudally arrange the electrodes 56 in the mannerillustrated in FIG. 7 can be used to selectively block APs in neuralfibers. As shown in FIGS. 10 and 11, the changes in transmembranepotential (ΔV_(m)) of neural fibers in fiber bundles that are in thevicinity of the electrodes 56 of the percutaneous lead 48 aregraphically illustrated when electric fields are generated by theelectrodes 56 during the neurostimulation regimens. The neurostimulationregimens are discussed in the context of first and second fiber bundlesFB1 and FB2. In the illustrated examples, the first fiber bundle FB1 isthe closest fiber bundle to the electrodes 56, and the second fiberbundle FB2 is the next closest fiber bundle to the electrodes 56.

A conventional stimulation regimen that uses uniformly sized electrodeswill serve as a reference for the stimulation regimens performed inaccordance with the present inventions, and will thus be initiallydescribed with reference to FIG. 10. In this conventional stimulationregimen, the upper and lower electrodes E_(U), E_(L) are activated asanodes, and the center electrode E_(C) is activated as a cathode. In theillustrated embodiment, 50% of the total current (e.g., 2 mA) is beingsourced at each of the upper and lower electrodes E_(U), E_(L), and 100%of the total current (e.g., 2 mA) is being sunk at the center electrodeE_(C).

The depolarizing electric field generated by the center electrode E_(C)is sufficient to create APs in some of the neural fibers in the firstfiber bundle FB1. In other words, the depolarization threshold DPT hasbeen met for the first fiber bundle FB1 in the tissue adjacent thecenter electrode E_(C). The depolarizing electric field generated by thecenter electrode E_(C) is substantially weaker at the second fiberbundle FB2 and is below the AP-creating depolarization threshold DPT.The locus of stimulation is, therefore, defined by the portion of thedepolarizing electric field generated by the center electrode E_(C) thatis at or above the depolarization threshold DPT.

The upper and lower electrodes E_(U), E_(L), which are functioning asanodes in the stimulation regimen illustrated in FIG. 10, will createhyperpolarizing electric fields in the neural tissue adjacent the upperand lower electrodes E_(U), E_(L). When the electric field is at orabove the hyperpolarization threshold HPT, the neural fibers within theelectric field will block APs that were fired at other points along thefibers. It should be noted here that the magnitude of thehyperpolarization threshold HPT has been estimated to be about 2 to 8times the magnitude of the depolarization threshold DPT. Thehyperpolarizing electric fields generated by upper and lower electrodesE_(U) and E_(L) in the exemplary stimulation regimen are below thehyperpolarization threshold HPT at the first fiber bundle FB1. As such,APs in the fiber bundle FB1 that fired at points in the neural fibersadjacent to center electrode E_(C) will not be blocked at pointsadjacent the upper and lower electrodes E_(U), E_(L). Thehyperpolarizing electric fields generated by the upper and lowerelectrodes E_(U), E_(L) will, of course, be even weaker at the secondfiber bundle FB2.

In the conventional stimulation regimen described above, the generationof APs in the fibers within the second fiber bundle FB2 will require anincrease in the depolarizing electric field generated by the centerelectrode E_(C) over that illustrated in FIG. 10. There may be instanceswhere the generation of APs in the first fiber bundle FB1, whichnecessarily results from the creation of a depolarizing electric fieldthat is strong enough to meet the depolarization threshold DPT at thesecond fiber bundle FB2, may lead to undesirable outcomes (e.g.discomfort or undesirable reflexive activity) for the patient.

The SCS system 10 may be used to solve this problem by preventing APsgenerated in the first fiber bundle FB1 from reaching the brain or endorgan. Specifically, as illustrated in FIG. 11, one example of astimulation regimen in accordance with the present invention involvescreating local AP blocks that prevent APs created within a portion ofthe depolarizing electric field that is at or above the depolarizationthreshold DPT from traveling in both directions beyond the stimulationsite. The effective locus of stimulation is, therefore, the region ofneural fibers that are generating APs that are not blocked at otherportions of the stimulation site.

Here, in the same manner described above with respect to FIG. 10, theupper and lower electrodes E_(U), E_(L) are activated as anodes and thecenter electrode E_(C) is activated as a cathode. However, the amount ofcurrent sunk at the center electrode E_(C) is sufficient to create adepolarizing electric field that is strong enough to meet thedepolarization threshold DPT at the second fiber bundle FB2 and causefibers within the second fiber bundle to generate APs. Such adepolarizing electric field will, of course, also cause the fibers inthe first fiber bundle FB1 to generate APs.

However, at least a substantial portion of the APs in the first fiberbundle FB1 will be prevented from passing electrode EU by thehyperpolarization. In particular, at least a substantial portion of theAPs (i.e., >10-20%) are blocked by hyperpolarizing tissue in the firstfiber bundle FB1, located on opposite sides of the tissue in the firstfiber bundle FB1 that is generating the APs, to at least thehyperpolarization threshold HPT. This may be accomplished bysignificantly increasing the level of current sourced from the upper andlower electrodes E_(U), E_(L), as compared to the level illustrated inFIG. 10 (e.g., about 2.5 mA each), in order to reach thehyperpolarization threshold HPT within the first fiber bundle FB1 at theupper and lower electrodes E_(U), E_(L).

Notably, if the sizes of the electrodes E_(U), E_(C), E_(L) were thesame, sinking all of the current sourced by the upper and lowerelectrodes E_(U) and E_(L) at the center electrode E_(C) could result ina depolarizing electric field that would meet or exceed thedepolarization threshold DPT in fiber bundles well beyond the secondfiber bundle FB2. However, because the center electrode E_(C) has anincreased tissue contacting surface area, the decreased current densitywill compensate for the increased current at the center electrode E_(C),thereby allowing the intensity of the depolarizing electric fieldcreated by the center electrode E_(C) to be reduced to a level where thedepolarization threshold DPT will not be met in fibers beyond the secondfiber bundle FB2.

Alternatively, rather the blocking AP in both directions, thestimulation regimen may involve locally blocking APs in a singledirection generated in the first fiber bundle FB1. Here, only one ofupper and lower electrodes E_(U), E_(L) is activated as an anode. Inthis case, 100% of the total current is being sourced at the upperelectrode E_(U) or lower electrode E_(L), and 100% of the total currentis being sunk at the center electrode E_(C).

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. A method of providing therapy to a patient,comprising: placing a plurality of electrodes adjacent to tissue of thepatient, the electrodes including first and second electrodes, the firstelectrode having a first tissue contacting surface area and the secondelectrode having a second tissue contacting surface area greater thanthe first tissue contacting surface area; and simultaneously sourcinganodic electrical current from the first electrode to a first tissuesite, thereby increasing an action potential threshold at the firsttissue site, and sinking cathodic electrical current from a secondtissue site to the second electrode, thereby providing the therapy tothe patient.
 2. The method of claim 1, wherein electrical current isconveyed between the first and second electrodes to source the anodicelectrical current from the first electrode and sink the cathodicelectrical current to the second electrode.
 3. The method of claim 1,wherein the second tissue contacting surface area is at least twice thefirst tissue contacting surface area.
 4. The method of claim 1, whereinthe current density on the first tissue contacting surface is greaterthan the current density on the second tissue contacting surface.
 5. Themethod of claim 1, wherein the anodic electrical current and cathodicelectrical current comprises a plurality of electrical pulses.
 6. Themethod of claim 1, wherein the tissue is spinal cord tissue.
 7. Themethod of claim 6, wherein the electrodes are arranged medio-laterallyalong the spinal cord tissue.
 8. The method of claim 7, wherein thesecond tissue site is located on dorsal column neural fibers of thespinal cord tissue, the first tissue site is located on dorsal rootneural fibers of the spinal cord tissue, the sunk cathodic electricalcurrent generates action potentials in the dorsal column neural fibersto provide the therapy to the patient, and the sourced anodic electricalcurrent increases the action potential threshold of the at least onedorsal root neural fiber.
 9. The method of claim 6, wherein theelectrodes are arranged rostro-caudally along the spinal cord tissue.10. The method of claim 9, wherein each of the first and second tissuesites are located on dorsal column neural fibers of the spinal cordtissue, the second electrode is a first distance from the first tissuesite and is a second greater distance from the second tissue site, thesunk cathodic electrical current generates action potentials at thefirst tissue site, and generates additional action potentials at thesecond tissue site to provide therapy to the patient, and the sourcedanodic electrical current blocks at least some of the action potentialsat the first site.